The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
(1) Field of the Invention
This invention generally relates to a device for increasing the efficiency of parametric sonar. More particularly this device utilizes characteristic effects of a cavitating transducer and alternatively introduce an outside stimulant to enhance the non-linear effects of a transmission medium.
Parametric sonar is well known. FIG. 1 shows a typical parametric sonar 10 positioned in a liquid environment 12. A first transducer 14 and a second transducer 16 are provided in acoustic communication with the environment 12. First and second transducers 14, 16 are joined with amplifiers 18 and 20, respectively. Amplifier 18 is joined to a first oscillator 22, and amplifier 20 is joined to a second oscillator 24. The oscillators 22, 24 are joined to a controller 26. In use, controller 26 activates first and second oscillators 22, 24 which provide a signal to the associated amplifier 18, 20 and then to the associated first transducer 14 and second transducer 16. The signal provided to first transducer 14 is at a first frequency, F1. This results in a first acoustic wave 28 at this frequency. The second transducer 16 receives a signal at a second frequency, F2, resulting in a second acoustic wave 30 at this frequency. Transducers 14 and 16 are oriented so that transmitted acoustic waves 28 and 30 overlap in an overlap region 32. In overlap region 32, an additive acoustic wave (not shown) having frequency, F1+F2, and a difference acoustic wave 34 having frequency, F1xe2x88x92F2, is created. Frequencies F1 and F2 are chosen so that the additive acoustic wave frequency dissipates over a short range while the difference acoustic wave 34 is transmitted at the desired range. Production of the difference acoustic wave 34 is very inefficient. Transducers 14 and 16 need to transmit a large amount of power in order to create a difference acoustic wave 34 having the desired power.
(2) Description of the Prior Art
The current art of parametric sonar takes advantage of the non-linearity associated with a transmission medium. It involves a generation of two frequencies, F1 and F2, which interact to form sum and difference frequency components. In a water medium, the sum frequency components (and the F1 and F2 components) quickly attenuate leaving only the difference frequency components. The main advantage of parametric sonar is that the beam width is based on F1 and F2 (not the difference frequency F1xe2x88x92F2), so that very narrow beams can be generated at low frequencies (even with a small aperture). One of the main disadvantages of parametric sonar in water is that the efficiency is very low, leading to a reduction in source level that can typically be 30 dB or more.
The following patents, for example, disclose parametric sonar devices utilized underwater:
U.S. Pat. No. 3,870,988 to Turner;
U.S. Pat. No. 3,882,444 to Robertson; and
U.S. Pat. No. 3,964,013 to Konrad.
Specifically, Turner discloses an underwater detection and identification method and apparatus utilizing the principle of parametric cross-modulation of ultrasonic frequencies within a non-linear propagation medium for obtaining an acoustical signature of an object under observation. The object is illuminated by ultrasound of suitable, high frequency projected from the observation platform and echo signals are received composed of side bands generated by combining the illuminating frequency with the relatively low signature frequency. The received ultrasonic side band frequency signals are then processed electronically to yield a signal representative of a characteristic of the object. The apparatus is essentially a hybrid, active-passive sonar operating in a continuous uninterrupted mode.
The patent to Robertson discloses a system for detecting and isolating incoming acoustic waves. The system includes means for transmitting a random noise signal that will intersect the incoming waves. Cross modulation products, particularly the first order sum and difference frequencies, occurring in the volume where the incoming low frequency and transmitted high frequency signals meet and intersect are propagated back toward a receiver where the modulated noise signals are correlated with the transmitted noise signal to isolate the lower frequency incoming signal. The interaction between the transmitted and incoming signals takes place at a plurality of volumetric segments which are located at various distances from the transmitter. By correlating the modulated return signals, which are received at selected intervals, with properly delayed replicas of the transmitted signal, the interaction, or cross modulation products, at any selected range can be isolated in the receiver. By summing these isolated signals, the incoming frequency can be detected, the overall system acting as a virtual receiving array.
Konrad discloses a cavitating parametric underwater acoustic source for generating acoustic energy at low and medium frequencies. The source comprises a plurality of electro-acoustic transducer elements which are electrically energized in a liquid medium such as water at two or more primary frequencies. Changes in the ambient liquid pressure at or adjacent the transducer cause cavitation in the liquid medium which produces a high degree of non-linearity resulting in the generation of sum and difference frequencies of the primary frequencies in the, liquid. The difference frequency is used to transmit acoustic energy in the liquid medium.
It should be noted that Konrad ""013 uses the same transducers to provide cavitation bubbles that are used to create the difference acoustic wave. Use of a transducer to create the large amplitude acoustic waves that are needed for cavitation can damage the transducer. Furthermore, control of low amplitude transducers is more precise for signal transmission.
Therefore it is an object of this invention to provide parametric sonar having increased efficiency in the transmission medium.
Another object of this invention is to provide parametric sonar having increased efficiency in the transmission medium by utilizing cavitation bubbles to increase the non-linearity of the transmission medium.
Still another object of this invention is to provide cavitation bubbles in a transmission medium in response to driving transducers at a power sufficient to generate the cavitation bubbles.
Yet another object of this invention is to provide parametric sonar having independently introduced bubbles in the transmission medium at a location of the projecting transducers to increase the non-linearity of the transmission medium.
In accordance with one aspect of this invention, there is provided a parametric sonar source operating in a fluid transmission medium. An improvement is provided for selectively increasing the efficiency of signals generated by transducers of the parametric source. This improvement includes an acoustic cavitation wave generated to intersect the acoustic waves emitted by the transducers of the parametric source. Interaction of the frequencies F1 and F2 of the acoustic transducer waves with the acoustic cavitation wave will generate subharmonics having a greater amplitude than in an absence of the acoustic cavitation wave. Preferably, the acoustic cavitation wave is introduced at a right angle or transverse to the acoustic waves emitted by the transducers of the parametric source, thereby providing an enhanced parametric sonar device.